Optical fibers are well known in the art and useful for many applications in modern communications systems. A typical fiber optic communications system comprises a source of optical signals, a length of optical fiber coupled to the source for transmitting the signals, and a receiver coupled to the fiber for receiving the signals. Basically, the optical fiber comprises an inner core fabricated from a material having a certain index of refraction, and a cladding surrounding the core. The cladding is comprised of a material having a lower index of refraction than the core. As long as the refractive index of the core exceeds that of the cladding, a light beam propagated along the core exhibits total internal reflection, and it is guided along the length of the core. One or more amplifying systems may be disposed along the fiber for amplifying the transmitted signal.
Filters and attenuators are useful in these systems to change the power levels of various signals. In modern communications systems, variable attenuators are becoming increasingly more important, especially in dense wavelength-division multiplexed (DWDM) systems. Variable attenuators are used to vary the amount of loss that light will experience as it passes through the system, ranging from low loss (&lt;1 dB), to very high loss (&gt;30 dB). The mechanism by which the attenuators induce loss in the signals may be attributable to coupling loss between fibers, polarization loss, absorption loss, scattering loss, or any combinations of these.
Variable attenuators typically have comprised complicated structures with moving parts that rotate or otherwise move the position of the fibers or of a separate attenuator device, requiring a continuous application of power to maintain a particular loss level. For example, U.S. Pat. No. 5,745,634 to Garrett, et al., "Voltage Controled Atenuator," issued Apr. 28, 1998, shows a variable attenuator with which the variation in attenuation is obtained by actuating a DC motor which displaces the position of the attenuator. U.S. Pat. No. 5,677,977 to Smith, "Optical Attenuator," issued Oct. 14, 1997, shows a variable attenuator with which the variation in attenuation is obtained by providing a circular loop of optical fiber which is rotated with use of a lockable rotating shaft clamped to the side of the loop. U.S. Pat. No. 5,781,341 to Lee, "Mororized Tunable Filter and Motorized Variable Attenuator," issued Jul. 14, 1998, shows a variable attenuator with use of a cam attached to a collimator; the cam rotates the collimator to adjust the loss.
A variable attenuator based on coupling loss is typically composed of two separate fibers whose separation is controlled with mechanical motion. As the amount of the separation between the fibers increases, the amount of loss also increases. See, for example, Brenner et al., "Low-Reflectivity In-Line Variable Attenuator Utilizing Optical Fiber Tpers," J. LIGHTWAVE TECH., Vol. 18 (1990), at p. 7, and U.S. application Ser. No. 09/097,549, "Magnetically Controlled Variable Optical Attenuators," filed by Espindola et al. on Jun. 15, 1998, and assigned to the present assignee, which is incorporated herein by reference. See also U.S. Pat. No. 5,319,733 to Emmons et al., "Variable Fiber Optical Attenuator," issued Jun. 7, 1994, which shows a variable attenuator using two terminated fibers that are placed in holders with their terminal ends aligned; the holders are rotated relative to each other while the alignment is maintained to provide variable attenuation. Variable attenuators based on polarization loss are typically composed of GRIN lenses to collimate light from the fiber, a plate or cell to rotate the polarization of light; and a polarizer to induce loss. See, e.g., U.S. Pat. No. 5,727,109, to Pan et al., "Optical Attenuator with Low Polarization Mode Dispersion," issued Mar. 10, 1998, and assigned to E-tek Dynamics, Inc.
As can be seen, variable attenuators typically have involved use of moving parts, with the waveguide itself being moved, and often they have required application of a continuous power supply to maintain a particular loss level. As may be appreciated, those concerned with the development of optical communications systems continually search for new components and designs including new attenuator designs. As optical communications systems become more advanced, there is growing interest in increasing the number of wavelengths that may be transmitted by the systems and in new methods and devices for modulating, filtering, and switching wavelength channels. The instant invention provides a variable attenuator device that may be used to achieve a predetermined amount of loss without a continuous supply of power to maintain a particular loss level and a higher speed of attenuation as compared with previous devices. Further advantages may appear more fully upon considering the description given below.